Step motors are used in a wide variety of applications that require precise motion control such as in printers, scanners, x-y tables, turntables, tape and disk drive systems, security cameras and other optical equipment, robotics, CNC (computer-numeric-control) machine tools, dispensers, and injector pumps. There have been many step motor designs introduced to achieve specific performance goals, such as reduced noise and vibration, increased resolution and accuracy of motor positions, and adequate holding torque and efficient power usage over a rare of motor speeds. Different modes of driving a step motor are available. The microstepping mode has been devised both to increase resolution of motor positions and to reduce vibration over other drive modes (full-stepping and half-stepping). Step motor design can be optimized to improve position accuracy when microstepping.
Varying the pitch angles of the stator teeth is the most common way to improve microstepping accuracy. The pitch angle of the rotor teeth must be constant it order to maintain a consistent step angle, so only the positions of the stator teeth are altered. Design work has also been done on the relationship between the rotor tooth width and stator tooth width, e.g., to achieve adequate holding torque, or to adjust for the particular stator tooth pitch design being used. In order to get the highest torque stiffness, the rotor tooth width should equal the stator tooth width for a one-phase ON operation; the rotor tooth width should be one-half of the stator tooth width for a full two-phase ON operation Because microstepping of a motor includes both one-phase ON and full two-phase ON conditions, as well as a wide range of intermediate conditions, at different motor positions, the relationship between the rotor tooth width and stator tooth width is chosen as a compromise to ensure adequate torque at all possible micro-step positions. For maximum microstepping accuracy, a sinusoidal torque profile is desired. A typical design might have a rotor tooth width approximately 3/4 of the stator tooth width, with the exact value chosen being dependent on factors such as tooth geometry and the stator tooth pitch design. Other designs might use a rotor-to-stator tooth width ratio of approximately 1/4.
While microstepping reduces noise and vibration over other drive modes, there still tends to be some remaining erratic motion when the motor passes through a one-phase ON position, which is a stable detent position of the motor. This is known as the zero-crossing problem in step motor design. In a one-phase ON position full (100%) current is applied to one set of stator coils, while another set of coils is at a zero crossing point with no (0%) applied current. The rotor and stator teeth are at maximum alignment at this stable position. The rotor is very easily pushed into this natural detent position, but has greater difficulty pulling out from the position. This typically results in erratic jerks in rotor motion. One common solution is to reduce the rotor tooth width. However, while it is acceptable for full-stepping or half-stepping motors, this solution also produces a non-sinusoidal torque profile, and thereby causes uneven micro-steps. A solution for accurately and precisely microstepping motors is sought.
U.S. Pat. No. 6,791,223 to Suzuki et al. describes a low vibration step motor in which the rotor unit has a developed pattern of alternate S and N poles magnetized on its circumference. The widths of the S poles are set to be different from the widths of the N poles, while each pair of adjacent S and N poles is set to a predetermined constant value.
U.S. Pat. Nos. 5,969,454 and 6,028,385 to Pengov et al. describe respective two-phase and three-phase switched reluctance motors, comprising a stator having evenly spaced stator poles, with windings for two or three phases wound about the stator poles, and a rotor having at least two rotor sections. A first rotor section includes a number of wide rotor poles, while a second rotor section includes a number of narrow rotor poles. During each phase energization, the rotor is advanced in a two-step fashion. In a first step, the leading edge of the wide rotor poles interact with first energized stator poles to induce a first torque on the rotor and produce a first angular rotation of the rotor. Then, in a second step, the narrow rotor poles are drawn into alignment with second energized stator poles to induce a second torque and produce a second angular rotation of the rotor.